Photodiodes and solar cells are often characterized by quantum efficiency (QE), which is generally expressed in units of outgoing electrons per incident photon.
Historically, apparatuses used to measure QE have used a conventional broadband lightsource such as quartz tungsten halogen, xenon arc, or metal halide, where the light is spectrally resolved with either a wavelength scanning monochromator or a set of bandpass filters. QE measurements are made sequentially as the apparatus is mechanically stepped through a series of predetermined wavelengths by adjusting either the monochromator diffraction grating angle or placing individual bandpass filters into the beam of broadband light. Alternatively, monochromator-based systems offer adjustable wavelength resolution and essentially continuous coverage over the range of interest. For solar cells, the range of interest can include wavelengths from the shortest solar emissions around 300 nm to the wavelength corresponding to the smallest bandgap present in the active region of the device, for example approximately 900 nm for cadmium telluride, and longer for some other materials. However, when scanning such a large wavelength range, monochromators require the use of order-sorting filters to prevent higher order diffraction from reaching the output slit, and also leak a measurable amount of broadband stray light that can not readily be removed by filters.
In these systems, a basic limitation is the speed with which the system can scan through a set of wavelengths. The long measurement time of conventional QE systems prevents the cell from being tested at multiple points of the cell to study localized or spatially varying effects. However, knowing the QE of a cell at different points can allow for better understanding of the films. For example, when referring to a cadmium telluride (CdTe) based photovoltaic device including both a CdTe layer and a cadmium sulfide (CdS) layer, a QE curve can show the CdS thickness. CdS absorbs at about 520 nm and below, whereas CdTe absorbs at ˜850 nm and below. By comparing the QE at certain wavelengths, for example at 600 nm and at 450 nm, the relative CdS thickness can be estimated.
As such, a need exists for a system and method capable of simultaneously measuring the QE at different points within a photovoltaic cell.